You might want to look at what you are doing to get infected and reinfected.
As for being sure, you never can be since malware needs to be detected before it is known so the bad guys tend to have the edge for a short while all the time. But if you have run a well respected detector then you should feel reasonably ok with your PC. Or run a number of them.
If really worried, reformat the drives and reload the operating system from scratch, and don't connect too the internet or use removable media with it again. But I'd say that was an overkill.